Not applicable.
The present invention relates to an improved laser transmitter and, more particularly, to a laser transmitter and method of laser transmitter compensation in which thermally induced errors in the grade of the projected beam of laser light are reduced by monitoring the transmitter temperature and correcting the transmitter operation accordingly.
Laser transmitters are commonly used in surveying and in the construction industry for measuring or checking elevations, grade, dimensions from off-set lines, and the like. It is well known, for example, to use a laser beam transmitter in place of the level instrument. At the location where elevation is to be measured or checked, a target or laser beam detector is employed to intercept the laser beam from the transmitter. The laser transmitter includes rotating optical components which produce a beam that sweeps in a generally horizontal plane. Some such transmitters incorporate visually readable level vials and manually adjustable screw mechanism to permit the transmitter to be oriented so that the plane defined by the beam is level or is tilted in a desired direction at a desired grade.
While such systems provide many improvements over the conventional level and rod survey equipment, they also present certain disadvantages and limitations. For example, the degree of accuracy in establishing a horizontal beam plane is dependent on the operator""s skill and judgement in reading the level vials as he operates the adjusting screws. Moreover, where the operator moves away from the device to tend the target or a beam detector, the laser beam transmitter can move out of level adjustment, as from being jarred, without the operator""s knowledge so that subsequent measurements are erroneous.
A laser transmitter having significant advantages over earlier prior art devices is shown in U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando et al, which is commonly assigned with the present application. The system disclosed in the Rando et al patent is one in which orientation of the laser beam reference plane is accomplished automatically. A support frame for the laser source is pivotally mounted on the base frame of the Rando et al device. The support frame carries electrical sensor vials which sense the orientation of the support frame and provide electrical signals used by a feedback control system. The feedback control system activates electric motors to move support frame into a position in which the vials are leveled. The vials are mounted on the support frame in such a manner that their positions may be adjusted by separate grade motors. When the reference laser plane is to be oriented at an angle to the horizontal, at least one grade motor is actuated by the operator to tilt a vial with respect to the support frame. The feedback control system then reorients the support frame to bring the vial back into its level position, tilting the frame by the desired amount. Other laser transmitters that incorporate level vials to detect orientation of transmitter components are shown in U.S. Pat. No. 5,852,493, issued Dec. 22, 1998, to Monnin, and in U.S. Pat. No. 6,055,046, issued Apr. 25, 2000, to Cain.
While providing a significant improvement over the prior art, it has been found that laser transmitters of this type may experience significant errors as a result of changes in ambient temperature. It has been found that a major source of these temperature induced errors are the level vials. A level vial of the type used in such transmitters typically comprises an electrically nonconductive vial casing, usually made of glass, that defines an elongated, arcuate chamber which curves generally downward toward its opposite ends. A quantity of electrically conductive fluid is provided in the chamber. Such a fluid may, for example, have a ketone component. A pair of end electrodes electrically communicate with the upper portions of the chamber adjacent its opposite ends and extend toward the central portion of said chamber. A common electrode extends the entire length of the chamber along its lower surface. The quantity of electrically conductive fluid in the chamber is such that an air bubble is left in the chamber, rising to whatever portion of the chamber is uppermost. It will be appreciated that, as the vial is tilted in one direction, the electrical impedance of a path from one end electrode through the electrically conductive fluid to the common electrode will increase, while the electrical impedance of a path from the other end electrode to the common electrode will decrease. When the vial is tilted in the opposite direction, the end-electrode-to-common-electrode impedances change in the opposite fashion. When the two end-electrode-to-common-electrode impedances are equal, the vial can be said to be oriented horizontally. It will be appreciated, however, that other impedance ratios might be defined as horizontal, if desired.
In any event, changes in the ambient temperature of a vial may cause the vial casing to change dimensions and shape. Of particular concern is any asymmetric change in the shape of the chamber, in that this may result in a change in the position of the air bubble and a change in the impedance ratio without any actual change in vial orientation. Vials have, in the past, been thermally insulated. While this reduces short term temperature fluctuations and temperature gradients along the length of the vial, it does not reduce errors stemming from asymmetric changes in chamber shape. A need exists for a laser transmitter in which such errors are eliminated, or at least minimized.
This need is met by a transmitter for projecting a beam of laser light and a method according to the present invention in which thermally induced errors are compensated. The transmitter includes a source of laser light, a projection arrangement for directing the laser light at a selected grade, a temperature sensor, and a temperature correction circuit. The projection arrangement includes a level vial which provides an electrical signal indicating that the laser light is being projected at the selected grade. The temperature sensor detects the temperature of the vial. The temperature correction circuit is responsive to the temperature sensor, and adjusts the projection arrangement such that temperature induced errors in the direction of the beam of laser light are compensated, and the beam of laser light is directed substantially at the selected grade.
The level vial comprises an electrically nonconductive vial casing defining an elongated chamber which curves generally downward toward opposite ends thereof, a quantity of electrically conductive fluid in the chamber, a pair of end electrodes electrically communicating with the upper portions of the chamber adjacent opposite ends and extending toward the central portion of the chamber, and a common electrode electrically communicating with the lower portion of the chamber. The vial is presumed to be level when resistances measured at each end are equal. The magnitudes of these resistances change with temperature, thus providing an indication of temperature. The temperature sensor includes a current sensor circuit for sensing the resistivity provided by the electrically conductive fluid. The current sensor includes a test resistance connected to one of the end electrodes, and a test circuit for determining the voltage across the test resistance. A test signal of predetermined voltage and short duration is applied across the end electrodes of the level vial and the voltage across the test resistance is measured. This measured voltage across the test resistance is an indication of the level of current through the level vial and this, in turn, is a function of vial temperature. In other words, the temperature of the vial is detected by detecting the bulk resistivity of the electrically conductive fluid in the vial.
The projection arrangement for directing the laser light at a selected grade may include an arrangement for changing the direction of the beam until the selected grade is reached, as indicated by the level vial electrical signal. The temperature correction circuit may include a circuit for providing an offset grade value to the arrangement for changing the direction of the beam. The circuit for providing an offset grade value to the arrangement for changing the direction of the beam may include a look-up table having offset grade values associated with specific temperature ranges. Preferably the look-up table may have offset grade values associated with two specific temperature ranges. These offset grade values are unique to the specific transmitter.
The projection arrangement may include a rotating optical element for directing the laser light in a rotating beam. The projection arrangement may also include first and second level vials, the first level vial providing an electrical signal indicating that the laser light is being projected at a selected grade in a first direction, and the second level vial providing an electrical signal indicating that the laser light is being projected at a selected grade in a second direction, orthogonal to the first direction. The first and second level vials may each comprise an electrically nonconductive vial casing defining an elongated chamber which curves generally downward toward opposite ends thereof, a quantity of electrically conductive fluid in the chamber, a pair of end electrodes electrically communicating with the upper portions of the chamber adjacent opposite ends and extending toward the central portion of the chamber, and a common electrode electrically communicating with the lower portion of the chamber. The temperature sensor may include a current sensor circuit for sensing the resistivity of the electrically conductive fluid in both of the first and second level vials. The current sensor circuit may include a test resistance connected to one of the end electrodes of each of the first and second vials, and a test circuit for determining the voltage across the test resistance in response to the application of a test signal of predetermined voltage and short duration across the parallel connected end electrodes of the first and second level vials.
The method of calibrating the transmitter comprises the steps of: a.) selecting a plurality of temperatures at which correction will be made; b.) subjecting the transmitter to an ambient temperature equal to the first temperature for a period sufficient to achieve thermal equilibrium and bring the level vial to the ambient temperature; c.) setting the transmitter at a specified grade; d.) measuring the actual grade of the laser; e.) determining the error in the grade achieved; f.) determining the grade offset needed to correct for the measured error; g.) measuring the temperature of the vial at that ambient temperature; h) storing the grade offset and the corresponding vial temperature for the first temperature in a look-up table; and i.) repeating steps a.) through h.) for each of the others of the plurality of temperatures. The step of selecting a plurality of temperature for which correction will be made includes the step of selecting three temperatures.
Accordingly, it is an object of the present invention to provide a laser beam transmitter which corrects effectively for thermally induced grade errors; to provide such a transmitter in which the temperature of the transmitter is assessed by monitoring the resistivity of a level vial that forms a part of the projection arrangement of the transmitter; to provide such a transmitter in which the correction is effected through grade offset values that are stored in a look-up table and that are unique to the errors of the specific transmitter; and to provide a method of compensating such a laser beam transmitter in which grade offset values are determined specifically for the transmitter of interest and stored in a look-up table. Other objects will be apparent from reference to the accompanying description and claims.